This invention relates to epoxy resin compositions which are easily moldable, have a low moisture permeability and reliability in the cured state, and are suitable for encapsulating semiconductor chips and especially as premolded hollow semiconductor packages. It also relates to premolded hollow semiconductor packages encapsulated with the epoxy resin compositions in the cured state.
In the electrical and electronic fields, epoxy resins featuring mechanical strength, moisture resistance and moldability find a variety of applications as insulating materials, laminates, adhesives and semiconductor encapsulants.
As opposed to essentially moisture impermeable metals and ceramics, epoxy resins classified as thermosetting resins have a coefficient of moisture diffusion. Upon exposure to humid conditions, epoxy resins absorb moisture and allow moisture to permeate therethrough. The moisture permeability of thermosetting resins often becomes a problem when they are used in the application requiring hermetic and water-proof seals, for example, in precision machines such as watches and electronic calculators, and electronic parts such as semiconductor packages, especially solid state imaging device (generally known as CCD) hollow packages and quartz oscillator hollow packages. For example, resin packages of CCD slowly absorb moisture when exposed to a hot humid environment for an extended period of time, though not in direct contact with water. If moisture is introduced into the hermetic space in excess of the saturated steam amount determined from the saturated water vapor pressure, moisture condenses. The device becomes inoperable by dew condensation.
An improvement in moisture permeability has long been desired for prior art epoxy resin compositions comprising an epoxy resin, a curing agent and an inorganic filler for use in hollow packages. One known means is to add an inorganic desiccant to the epoxy resin composition so that the desiccant adsorbs moisture permeating through the cured item, preventing moisture from entering the hollow interior.
JP-A 8-157694 of the same assignee as the present invention discloses an epoxy resin composition comprising at least 10 parts by weight of an inorganic desiccant per 100 parts by weight of an epoxy resin and a curing agent combined. Specifically, using AMT silica by Mizusawa Chemical K.K. as the inorganic desiccant, the composition is rendered low moisture permeable.
AMT silica, however, has a low true specific gravity and contains a relatively large amount of ionic impurities since it is porous silica obtained by sintering zeolite. The low true specific gravity means that a certain weight of silica added accounts for a larger volume so that an increased amount of silica can obstruct flow. The large amount of ionic impurities interfere with the curing function of the curing catalyst so that cure may become short, resulting in drops of hot strength and bond strength.
JP 2,750,254 (JPA 6-232292) discloses a semiconductor package comprising an insulating substrate containing 0.1 to 50% by weight of a desiccant. The amount of desiccant added to an epoxy resin composition should preferably be increased since a small amount of desiccant does not fully adsorb moisture penetrating through the cured item. Illustratively, in a moisture permeation reliability test, a package sample containing a small amount of desiccant allows moisture to reach the cavity so that moisture may condense on the glass lid. This drawback can be eliminated by increasing the amount of desiccant. However, since conventional desiccants are poorly compatible with epoxy resins and curing agents, they exacerbate the flow particularly when added in large amounts. The epoxy resin composition with retarded flow can cause such defects as short shots and voids when molded by a transfer molding machine, and is thus unsuitable for semiconductor encapsulating purposes.
An object of the invention is to provide an epoxy resin composition which is flowable and easily moldable and cures into a product having a low moisture permeability, and a premolded hollow semiconductor package encapsulated with the epoxy resin composition in the cured state.
We have found that an epoxy resin composition comprising an epoxy resin, a curing agent, and an inorganic filler flows smoothly and is easily moldable when the filler is porous silica having a specific surface area of 6 to 200 m2/g, a true specific gravity of 2.0 to 2.2, and a mean particle size of 2 to 50 xcexcm. The composition cures into a product of quality having a low moisture permeability. In summary, the composition is smoothly flowable and readily bondable and curable into a product having minimized moisture permeation. The composition is thus suited for forming premolded hollow semiconductor packages.
Accordingly the invention provides an epoxy resin composition comprising an epoxy resin, a curing agent, and an inorganic filler in the form of porous silica having a specific surface area of 6 to 200 m2/g, a true specific gravity of 2.0 to 2.2, and a mean particle size of 2 to 50 xcexcm. In some preferred embodiments, the porous silica has been prepared by forming a silica gel having a weight average particle size of up to 50 xcexcm by a sol-gel process, and firing the silica gel at a temperature of 700 to 1,200xc2x0 C.; the porous silica has a moisture pickup of at least 0.3% by weight when kept at 25xc2x0 C. and RH 70% for 24 hours; the porous silica contains up to 1 ppm of each of alkali and alkaline earth metals. The porous silica preferably accounts for 40 to 90% by weight, more preferably at least 55% by weight of the entire epoxy resin composition.
The invention also provides a premolded hollow semiconductor package encapsulated with a cured product of an epoxy resin composition comprising an epoxy resin, a curing agent, and an inorganic filler including a porous silica having a specific surface area of 6 to 200 m2/g, a true specific gravity of 2.0 to 2.2, and a mean particle size of 2 to 50 xcexcm.